N inductor system for controlling relative displacement between workpieces and a

ABSTRACT

A system is disclosed for controlling the intermittent feeding of workpieces in magnetically coupled relationship with respect to an induction heating coil. A pusher mechanism is provided and is reciprocated intermittently to push a workpiece to be heated into one end of the coil and to displace workpieces in the coil so that the workpiece adjacent the other end thereof is discharged. Intermittent reciprocating movement of the pusher mechanism is controlled by an electric signal representative of the energy input to the workpieces, and which signal is derived by creating a power signal representative of the instantaneous power supplied to a workpiece and integrating the power signal with respect to time. When the energy input signal reaches a preselected value the pusher mechanism is actuated and the time integrating portion of the control circuitry is interrupted and reset. The power signal is then again integrated with respect of time to create another energy signal for subsequently actuating the pusher mechanism.

Kasper et al.

Sept. 4, 1973 SYSTEM FOR CONTROLLING RELATIVE Primary Examiner-J. V.'Iruhe Assistant ExaminerB. A. Reynolds A tib'r'hey Taffies'HfTilbeiry",Alfred C. Body and Robert V. Vickers [75] Inventors: Robert J. Kasper,Seven Hills;

William H. Killian, Parma Heights, both of Ohio [57] ABSTRACT A systemis disclosed for controlling the intermittent [73] Asslgnee' g? stylesfeeding of workpieces in magnetically coupled relationeve an hlo shipwith respect to an induction heating coil. A pusher [22] Filed; Apr, 28,1972 mechanism is provided and is reciprocated intermittently to push aworkpiece to be heated into one end [21 1 Appl' 248423 of the coil andto displace workpieces in the coil so that the workpiece adjacent theother end thereof is dis- [52] US. Cl. 219/10.77, 219/10.73 gIntermittent r ipr ng m vem n f h [51] Int. Cl. H051; l/02 pusher ni m in r d by an elec ric signal [58] Field of Search 219/10.77, 10.75,representative of the gy input to the workpieces. 219/109, 110, 10,73;323/20 and which signal is derived by creating a power signalrepresentative of the instantaneous power supplied to [56] ReferencesCited a workpiece and integrating the power signal with re- UNITEDSTATES PATENTS spect to time. When the energy input signal reaches a2,510,770 6 1950 Bohn 219 10.77 x pmselecied f mechamsm afttuaied 3 240961 3/1966 Noth 219,110 x and the tlme integrating portion of thecontrol circuitry 313731330 3/1968 OBrien::.... $2113.. 323 20 x isinterrupted and reset The Power Signal is again 3,601,571 8/1971 Curcio219 10.77 x integrated with respect of time to Create another 2,971,7542/1961 Seyfried 219/1077 x y signal for subsequently at ng the pushermech- 3,389,239 6/1968 Treppa et al. 219/110 anism.

10 Claims, 1 Drawing Figure 1 F. AUTOMATIC POWER VOLTAGE FACTOR IO LMSL/REG. CONTROL 22 24 52- \u9fij j 1 w 32 34 I4 44 w A f p \O\\ OJ \(5\ 6 Q\0 \O\ 01 42 \pgi SYSTEM FOR CONTROLLING RELATIVE DISPLACEMENT BETWEENWORKPIECES AND AN INDUCTOR The present invention relates to the art ofinduction heating and, more particularly, to a system for controllingthe relative displacement between workpieces and an inductor inmagnetically coupled relationship with respect thereto.

While the present invention finds particular utility and will bedescribed in conjunction with the progressive induction heating of aplurality of metal workpieces as the latter are intermittently advancedrelative to a fixed inductor, it will be appreciated that the principlesof the invention have a much broader application and are applicable ingeneral to any progressive induction heating operation wherein there isa relative displacement between an inductor and workpiece or workpieces.

Induction heating apparatus has been provided heretofore for the purposeof progressively heating a plurality of workpieces such as billets forforgings as the workpieces are intermittently advanced in magneticallycoupled relationship with respect to an inductor of the apparatus.Generally, the apparatus includes a work feed mechanism which isintermittently actuated to move a cold workpiece into magneticallycoupled relationship with respect to one end of the inductor and to movethe workpieces ahead of the cold workpiece toward the other end of theinductor, whereby a heated workpiece is discharged from the inductoreach time a cold workpiece is introduced for heating. When the inductoris fully loaded with workpieces, and the thermal conditions of theinductor have reached a state of equilibrium, the workpieces areadvanced relative to the inductor at a uniform periodic rate so that theworkpieces discharged from the inductor are generally uniformly heatedto the extent desired.

While workpieces heated in the above manner generally are acceptableonce uniform thermal conditions are reached, uncontrollable conditionsare encountered during apparatus operation which often result inworkpieces being underheated or overheated. In this respect, theworkpieces are advanced by the feed mechanism at preset timed intervals.Thus, a variation in the voltage level of the power source for theinductor upwardly or downwardly from a preselected level will result,respectively, in overheating and underheating of workpieces beingadvanced relative to the inductor during such variation. Likewise,variations in the size of workpieces being heated from a nominal sizeused as a basis for determining operating parameters of the apparatusresults in overheating or underheating, respectively, of an under sizeand over size workpiece. The actual dimensions of individual workpieceshaving the same nominal dimensions can vary considerably, especially ifthe workpieces are billets for forging. While the level of supplyvoltage to the inductor can be adjusted such as by the manipulation ofmanual dials, it will be appreciated that it would be practicallyimpossible, even for the most experienced operator, to successfullyadjust the power level and feed rate under such varying operatingconditions to avoid overheating or underheating problems. Specialvoltage control equipment can be employed, but this is expensive andserves only to maintain power supply voltage at a constant level,whereby indexing control is still necessary. Further,

variations in the operation of indexing timers employed can vary theworkpiece heating.

In addition to the problems encountered during operation of theinduction heating apparatus under stabilized thermal conditions,including the problems discussed above, there are further and moreserious problems encountered in and attendant to starting up of theapparatus such as at the beginning of a workday. The latter problemsresult in a considerable reduction of operating efficiency and loss ofproduction time and workpieces. In this respect, the inductor of theapparatus is cold. Presuming there are no workpieces positioned formagnetic coupling with the inductor, one or more workpieces are sopositioned relative to the input end thereof and the inductor isenergized and the feed mechanism operated to achieve intermittentadvancement of the workpieces relative to the inductor. Since theinductor is not fully loaded, a low power is drawn by the inductor whenthe inductor is energized at the rated voltage. Thus, the feed mechanismhas to be controlled to decrease the rate of indexing or advancement ofworkpieces in an effort to avoid underheating of the workpieces. As theapparatus warms up, the indexing rate must be increased to preventoverheating of workpieces, and the indexing adjustment must be continueduntil such time as thermal conditions of the inductor stabilize. Thetime required to reach the latter condi tion often exceeds considerablythe time required to fully load the inductor with workpieces between theinput and discharge ends thereof.

Varying of the feed rate during the warm-up period may be achieved by anoperator manually controlling actuation of the feed mechanism. Anydegree of accuracy obtained in changing the feed rate in this manner,however, can be attributed primarily to the operators experience becausehe bases his manual actuation of the feed mechanism on his observationof continually varying conditions of the apparatus and workpieces asthey are progressively heated. It will be appreciated therefore that theadjustment procedure is largely guesswork. Further, when the inductorbecomes substantially or fully loaded with workpieces before thermalconditions stabilize, it may be necessary in addition to feed control,to reduce the level of supply voltage to the inductor to avoid drawingexcess current. This adjustment may also be manual and if so, isperformed by the operator in response to observed conditions. Theforegoing conditions vary continuously during inductor warm-up and ithas been found from previous experience that even the most skilled andexperienced operator can not make the necessary adjustments with thedegree of accuracy required to prevent substantial loss of workpiecesdue to over or underheating. Thus, production time is lost and theefficiency of the induction heating process is reduced. Other efforts toavoid the foregoing problems following loading of the inductor withworkpieces include the use of temperature sensing devices operable tocontrol indexing of the feed mechanism in accordance with the sensedtemperature of a heated workpiece at the discharge end of the inductor.While such devices may provide for achieving better results than areachieved by manual adjustments made by an operator, it remains that lessthan desirable results are achieved. In this respect, changes in theindexing rate are based on the sensed temperature of a given workpieceat the discharge end of the inductor. If the voltage level varies fromthe rated or adjusted level the changes dictated by the temperature ofthe latter can result in under or overheating of workpieces in theinductor.

In view of the foregoing and other disadvantages and difficultiesattendant to the start-up and subsequent operation of induction heatingapparatus for the progressive heating of workpieces it will beappreciated that there is a need for a system for more accuratelycontrolling the indexing rate of workpieces. This need is fulfilled bythe present invention which provides for the feed mechanism of theapparatus to be actuated by an electrical signal representative of adesired energy input to a workpiece magnetically coupled with theinductor. This provides for the feed mechanism to be cycled duringstart-up and subsequent operation of the apparatus in accordance withthe actual energy applied to the workpieces. Thus, during start-up aminimum number of underheated parts are discharged from the apparatus ina minimum amount of time and during operation after thermal conditionshave stabilized a minimum number of underheated workpieces aredischarged.

More particularly, in accordance with the present invention, theinductor of the apparatus is connected to a power supply and a system isprovided for creating a power signal which is representative of theinstantaneous power supplied to a workpiece magnetically coupled withrespect to the inductor. The power signal is continuously integratedwith respect to time to create an energy signal representative of energyinput to the workpiece. When theenergy signal reaches a preselectedvalue it actuates the workpiece feed means such as a pusher mechanism tomove a cold workpiece into magnetically coupled relationship withrespect to the inductor and to advance workpieces ahead of the coldworkpiece toward the discharge end of the inductor. By integrating thepower signal with respect to time the energy applied to a givenworkpiece can be determined, and when the energy reaches the preselectedlevel the workpiece feed mechanism is actuated regardless of the timerequired for the signal to reach the preselected level. Thus, at eachindexing of the workpieces there is a given energy input to the inductorso that each workpiece in moving through the inductor receives the sametotal energy input. Any parameter changes during startup or subsequentoperation of the apparatus are compensated for by changes in the timerequired for the energy signal to reach the preselected level. Thisprovides an extremely accurate method for controlling the actuation ofthe workpiece feed mechanism and thus advantageously reduces the numberof underheated or overheated workpieces to a minimum. During start upand shut down operations automatic power factor control and voltageregulation is employed to limit current in the inductor.

An object of the present invention is the provision of a system forcontrolling the intermittent feeding of workpieces to be progressivelyheated relative to an inductor of an induction heating installation tominimize the loss of production time and workpieces.

Another object of the present invention is the provision of a system forcontrolling the intermittent feeding of workpieces relative to aninductor of an induction heating installation in accordance with theenergy input to a workpiece being heated.

Yet another object of the present invention is the provision of a systemfor controlling the intermittent feeding of workpieces relative to theinductor of an induction heating installation in accordance with anelectrical signal derived by generating a signal corresponding to thepower supplied to a workpiece and integrating the power signal withrespect to time.

A further object of the present invention is the provision of a systemof the foregoing character which provides for the workpiece feedmechanism to be intermittently actuated and controlled in accordancewith the actual energy input to a workpiece being inductively heated.

The foregoing objects, and others, will in part be obvious and in partmore fully pointed out hereinafter in conjunction with the descriptionof the accompanying drawing depicting a preferred embodiment of the present invention.

Referring now in greater detail to the drawing wherein the showings arefor the purpose of illustrating a preferred embodiment of the inventiononly and not for the purpose of limiting the same, an induction heatinginstallation is illustrated which includes a power supply 10, outputcircuit 12 and inductor 14 for inductively heating workpieces W. In theembodiment illustrated, power supply 10 is a motor generator set, andoutput circuit 12 includes leads 16 and 18 and variable capacitor 20connected thereacross for automatically adjusting the power factor ofthe power supply output. The output circuit further includes a couplingtransformer 22 having a primary winding 24 and a secondary winding 26.Secondary winding 26 is connected to leads 28 and 30 which are connectedto opposite ends of inductor l4. Inductor 14 may take any one of avariety of forms and, in the embodiment illustrated, is in the form of asolenoidal'coil comprised of a plurality of convolutions 32 embedded insuitable insulating refractory material 34. In a manner well known, theinductor is produced from tubular non-magnetic conducting material suchas copper and the tubular structure provides for a cooling fluid such aswater to be circulated through the inductor to cool the latter.

Inductor 14 is suitably supported relative to a floor or the like by anunderlying support 36, and workpiece support components 38 and 40 areprovided adjacent opposite ends of the inductor. Support component 38 isdisposed adjacent the input end of inductor l4 and is adapted to supporta cold workpiece to be heated, and support component 40 is disposedadjacent the output end of inductor 141 so as to receive a heatedworkpiece upon discharge thereof from induction heating relationshipwith inductor 14. It will be appreciated that suitable means, notillustrated, are provided in conjunction with support component 38 todeliver workpieces to be heated into a position for the workpiece to bemoved into magnetically coupled relationship with inductor 14 at theinlet end of the inductor, and that means will be provided inassociation with support component 40 to convey discharged heatedworkpieces away from the inductor.

A pusher mechanism 42 is supported adjacent the inlet and of inductor 14and is operable to displace a workpiece from support component 38 andinto the inlet end of the' inductor, whereby the workpieces in theinductor are advanced toward the discharge end thereof and the workpieceadjacent the discharge end is displaced onto support component 40. Inthe embodiment illustrated, the workpiece feed mechanism comprises areciprocable pusher member 44 in the form of a reciprocable shaft orpiston rod component of a hydrualic or pneumatic motor having a cylinder46 in which a piston member, not illustrated, is reciprocated inresponse to the flow of control fluid into and out of the cylinder. Theflow of fluid relative to cylinder 46 is controlled by a valve 48interposed between a control fluid source, not illustrated, and cylinder46. Valve 48 is electrically actuable and may, for example, be relaycontrolled. The valve has two modes of operation. In one of the modes ofoperation the valve is operable to control fluid flow to cylinder 46 ina manner whereby pusher member 44 moves outwardly of cylinder 46 towardsinductor 14. In the other mode of operation, valve 48 controls the fluidflow in a manner whereby pusher member 44 is retracted relative tocylinder 46. It will be appreciated that the valve will operate in oneof the modes when electrically actuated and in the other of the modeswhen unactuated. The particular structures of the valve and fluid motorare not pertinent to the present invention, and it will be appreciatedthat a number of suitable motor and valve arrangements could be providedto achieve the desired reciprocating movement for workpiece stepping.Moreover, it will be appreciated that feed mechanism defined by devicesother than fluid motor assemblies could be employed under the control ofan electrically actuated device other than a valve. For example, thereciprocating feed mechanism could be electrically driven such as by amotor under the control of an electrically actuated switch, or variouscombinations of electric and fluid devices could be employed to achieveworkpiece feed control.

In the progressive heating of a plurality of workpieces by an inductorsuch as inductor 14, it is desirable to control the intermittent feedingof workpieces to the inductor in a manner whereby the heating ofworkpieces relative to one another upon discharge from the inductor isuniform. The difficulty in achieving desired heating of workpieces isrealized during the start-up operation of an induction heatinginstallation when the inductor is cold and is either empty or filledwith cold workpieces, whereby a considerable number of underheated oroverheated workpieces are discharged from the inductor during thestart-up operation. In accordance with the present invention, however,the number of under or overheated parts is advantageously reduced to aminimum by providing for the workpiece feed mechanism to advanceworkpieces relative to the inductor in accordance with the energy inputto the workpieces. Thus, the advancement of workpieces relative to theinductor is automatically controlled in accordance with an extremelyaccurate measurement of the extent of workpiece heating thus toeliminate the guess work heretofore required with regard to varying theintervals between advancements of workpieces during a start-upoperation. Further, in accordance with the present invention, over orunderheating of workpieces during operation of the apparatus subsequentto start-up heretofore resulting from variations in the power supplyvoltage are substantially reduced. The end result is a minimizing oflost production time and workpiece wastage during and following start upand during shut down.

The desired workpiece feeding is achieved in accordance with the presentinvention by means of a control device 50 which is operable to controlactuation of valve 48 from one mode of operation thereof to the other inaccordance with measured energy input to a workpiece or workpiecesdisposed in magnetically coupled relationship with inductor 14. Controldevice 50 may take the form illustrated in the copending application ofGeorge D. Pfaffman, Ser. No. 185,316, filed Sept. 30, 1971 and assignedto the assignee of the present application, and the disclosure of thelatter application is incorporated herein by reference. Briefly, controldevice 50 is comprised of electronic components including means to sensethe voltage across primary winding 24 of transformer 22 and means tosense the current in the primary circuit. More particularly, controldevice 50 includes a pair of voltage sensing leads 52 and 54 connectedacross primary winding 24 and a pair of current sensing leads 56 and 58inductively coupled with lead 16 of primary winding 24. The sensedvoltage and current are coupled to a transducer (not illustrated) whichprovides an output signal representative of the instantaneous powersupplied to a workpiece or plurality of workpieces which are thenmagnetically coupled with inductor 14. This proportional output signalis connected to the input of control devices 50. Control device 50further includes a circuit for continuously integrating the power signalwith respect to time to create a signal which is representative of theenergy input per workpiece to the workpiece or workpieces. The magnitudeof the energy input signal varies in accordance with time and at anygiven time has a magnitude corresponding to the energy input perworkpiece to the workpiece or workpieces at that given time. Further, anelectric relay is connected to electrically actuated valve 48 through alead 60. By providing for this relay to be actuated in response to theenergy input signal reaching a preselected value corresponding to apredeterminedenergy input per workpiece, the workpiece feed mechanismwill be actuated each time the energy input per workpiece reaches thepreselected value. The desired energy input per workpiece, andaccordingly the level of the energy input signal at which the signalperforms its control function, is determined by the total energy inputto a workpiece required to achieve the desired heating thereof duringits movement through the coil. Thus, if five indexing steps are requiredto move the workpiece through the coil, the level of the energy inputsignal to cause indexing would be one fifth of the total energy inputrequired for the workpiece to be discharged in the desired heatedcondition.

Since the power signal is integrated with respect to time, changes orvariations in the power supply voltate from a desired level during andsubsequent to the startup operation are compensated for by the controldevice so that the feed mechanism is actuated only when the energy inputper workpiece reaches the preselected level and regardless of the timerequired for the signal to reach that level. Thus, presuming inductor 14to be empty at the beginning of the start-up operation, one or twoworkpieces will be introduced into the input end of the inductor and,since the load resistance is high, the inductor will draw low power whenstarted at rated voltage and unity power factor. The cycle time foractuation of the feed mechanism will therefore be longer than if theinductor were operating under stabilized thermal conditions. The powerlevel is increased as the inductor becomes loaded, whereby less time isrequired for the energy signal to reach the preselected value.Accordingly, the cycle time of the feed mechanism increases.Advantageously, the regulation of the power supply voltage level duringstart-up does not have to be achieved with a high degree of accuracy. Inthis respect, regardless of the variation in voltage from a given level,the power signal is integrated with respect to time, whereby the voltagevariations are compensated for. Thus, the control device functions tocause actuation of the feed mechanism only when the energy input reachesthe preselected value per workpiece for the workpieces being heated.

it will be appreciated, of course, that the time integrating circuitryof the control device must be reset following actuation of the feedmechanism to provide for the subsequent time integration of the powersignal necessary to produce another energy input signal for againactuating the feed mechanism. Such a resetting function can be achievedin any desired manner and, for example, a reset control lead 62 may beconnected to the integrating circuitry from power source lead 16 througha switch 64 adapted to be momentarily opened to reset the integratingcircuit upon actuation of the feed mechanism. It will be appreciatedthat the momentary opening of switch 64 can be achieved in any desiredmanner and could, for example, be achieved by physically associating theswitch with the feed mechanism so that the switch is momentarily openedupon movement of the feed mechanism in the workpiece feeding direction.

When the electrical and temperature conditions of the induction heatingapparatus become stabilized, the control device advantageously operatesto provide for the progressive heating of the workpieces to be achievedin a manner whereby succeeding discharged workpieces are more uniformlyheated relative to one another than heretofore possible. In thisrespect, any changes in operating parameters of the power supply systemsuch as a momentary increase or decrease in the power input iscompensated for by the time integrating circuitry, whereby the desiredenergy input per workpiece remains the same regardless of suchvariations.

Control device 50 can also be employed to advantage during unloading ofinductor 14 such as when it is desired to shut down the apparatus. llnthis respect, ceramic or brick slugs can be introduced into the inputend of the inductor in place of cold workpieces until such time as allof the workpieces in the inductor have been discharged from the oppositeend thereof. During such unloading, the load resistance progressivelyincreases and the control device operates to increase the cycle time ofthe feed mechanism so that the workpieces continue to be discharged fromthe inductor properly heated in accordance with the total energy inputrequired per workpiece. During such unloading, the voltage output of thepower source may be regulated and the power factor controlled inaccordance with the decrease in load reactance and the variations ofthese parameters do not have to be extremely accurate since the controldevice functions to integrate the power input signal in accordance withtime to derive the desired control signal for the feed apparatus.

With regard to regulation of the power supply voltage it will beappreciated the voltage level will have to be adjusted under certaincircumstances such as, for example, if the apparatus is to be started uploaded with cold workpieces left therein from a previous operation.Accordingly, voltage regulator means, not illustrated in the drawing,may be associated with the motor generator set for this purpose.Further, such voltage regulating means can be automatic and responsiveto a condition such as the temperature of workpieces discharged from theinductor so as to maintain the supply voltage within desired limits inresponse to a sensed condition. As mentioned hereinabove, the accuracyof such regulation is not critical to the workpiece feeding control.

While the inductor is described in the foregoing embodiment as being asolenoidal coil, it will be appreciated that the inductor can be ofother forms. Moreover, it will be appreciated that power supplies otherthan the motor-generator power supply illustrated can be employed. Stillfurther, it will be appreciated that the present invention is applicableto the intermittent advancing of a continuous workpiece relative to aninductor or to the heating of a continuous workpiece by a scanningmovement of the inductor relative thereto.

We claim:

1. In an induction heating installation including an inductor, a powersupply and an output circuit connecting the inductor to the powersupply, a system for controlling intermittent movement between theinductor and a workpiece in magnetically coupled relationship therewith,said system including means for producing a power signal representativeof the power input to the workpiece from the inductor, means forintegrating said power signal with respect to time to produce a signalrepresentative of energy input to said workpiece, means for displacingone of said workpiece and inductor relative to the other, and meanscontrolled by said energy input signal to actuate said displacing meanswhen said energy input signal reaches a preselected value.

2. The system according to claim 1, wherein said inductor is fixed andsaid workpiece is displacable in a given direction relative thereto.

3. The system according to claim 2, wherein said inductor has a lengthin said direction and said displacing means is operable to displace saidworkpiece a distance less than said length.

4. The system according to claim 1, wherein said inductor is fixed andsaid workpiece is displacable in a given direction relative thereto,said inductor having a length in said direction and said workpiecehaving a length in said direction less than the length of said inductor.

5. The system according to claim 4, wherein said inductor is a helicalcoil and said displacing means is operable to move said workpieceaxially with respect to said coil.

6. The system according to claim 5, wherein said displacing means has adisplacement in the axial direction generally equal to the length ofsaid workpiece.

7. Ar: inductive heating system comprising an induction heating coiladapted to receive a plurality of workpieces to be heated, a powersupply, an output circuit connecting said coil across said power supply,means to intermittently index said workpieces relative to said coil in adirection from one end thereof toward the other for said workpieces tobe progressively heated during movement through said coil, means toproduce a power signal representative of the power input per workpieceto said workpieces, means to integrate said power signal with respect totime to produce an energy signal representative of energy input perworkpiece, and means controlled by said energy signal to actuate saidindexing means when said energy signal reaches a predetermined value.

8. The system according to claim 7, and means responsive to actuation ofsaid indexing means to reset said integrating means.

9. An induction heating system comprising an inductor having oppositeends, a power supply, an output circuit connecting the opposite ends ofsaid inductor across said power supply, means to intermittently feedworkpieces relative to said inductor from one end thereof toward theother for workpieces to be in magnetically coupled relationship withsaid inductor, means to produce a power signal representative of thepower supplied to a workpiece magnetically coupled with said inductor,means for integrating said power signal with respect to time to createan energy signal representative of energy input to said workpiece, meanscontrolled by said energy input signal to actuate said feed means whensaid energy signal reaches a preselected value to displace saidworkpiece toward said other end of said inductor, and means to resetsaid integrating means in response to actuation of said feed means.

10. The system according to claim 7, wherein said workpieces each have alength in the direction of displacement less than the distance betweensaid opposite ends, and said feed means is operable to displace saidworkpiece a distance corresponding generally to the length thereof.

t l i

1. In an induction heating installation including an inductor, a powersupply and an output circuit connecting the inductor to the powersupply, a system for controlling intermittent movement between theinductor and a workpiece in magnetically coupled relationship therewith,said system including means for producing a power signal representativeof the power input to the workpiece from the inductor, means forintegrating said power signal with respect to time to produce a signalrepresentative of energy input to said workpiece, means for displacingone of said workpiece and inductor relative to the other, and meanscontrolled by said energy input signal to actuate said displacing meanswhen said energy input signal reaches a preselected value.
 2. The systemaccording to claim 1, wherein said inductor is fixed and said workpieceis displacable in a given direction relative thereto.
 3. The systemaccording to claim 2, wherein said inductor has a length in saiddirection and said displAcing means is operable to displace saidworkpiece a distance less than said length.
 4. The system according toclaim 1, wherein said inductor is fixed and said workpiece isdisplacable in a given direction relative thereto, said inductor havinga length in said direction and said workpiece having a length in saiddirection less than the length of said inductor.
 5. The system accordingto claim 4, wherein said inductor is a helical coil and said displacingmeans is operable to move said workpiece axially with respect to saidcoil.
 6. The system according to claim 5, wherein said displacing meanshas a displacement in the axial direction generally equal to the lengthof said workpiece.
 7. An inductive heating system comprising aninduction heating coil adapted to receive a plurality of workpieces tobe heated, a power supply, an output circuit connecting said coil acrosssaid power supply, means to intermittently index said workpiecesrelative to said coil in a direction from one end thereof toward theother for said workpieces to be progressively heated during movementthrough said coil, means to produce a power signal representative of thepower input per workpiece to said workpieces, means to integrate saidpower signal with respect to time to produce an energy signalrepresentative of energy input per workpiece, and means controlled bysaid energy signal to actuate said indexing means when said energysignal reaches a predetermined value.
 8. The system according to claim7, and means responsive to actuation of said indexing means to resetsaid integrating means.
 9. An induction heating system comprising aninductor having opposite ends, a power supply, an output circuitconnecting the opposite ends of said inductor across said power supply,means to intermittently feed workpieces relative to said inductor fromone end thereof toward the other for workpieces to be in magneticallycoupled relationship with said inductor, means to produce a power signalrepresentative of the power supplied to a workpiece magnetically coupledwith said inductor, means for integrating said power signal with respectto time to create an energy signal representative of energy input tosaid workpiece, means controlled by said energy input signal to actuatesaid feed means when said energy signal reaches a preselected value todisplace said workpiece toward said other end of said inductor, andmeans to reset said integrating means in response to actuation of saidfeed means.
 10. The system according to claim 7, wherein said workpieceseach have a length in the direction of displacement less than thedistance between said opposite ends, and said feed means is operable todisplace said workpiece a distance corresponding generally to the lengththereof.